Isthmal stem cells sustain intestinal homeostasis and regeneration

The currently accepted intestinal epithelial cell organization model equates crypt base columnar (CBC) cells, marked by high levels of Lgr5 expression, with the intestinal stem cell (ISC)[1][1]. However, recent intestinal regeneration studies have uncovered limitations of the ‘Lgr5-CBC’ model[2][2], [3][3], leading to two major views: one favoring the presence of a quiescent reserve stem cell population[4][4]–[7][5], the other calling for differentiated cell plasticity[8][6]–[11][7]. To test if an alternative model may help reconcile these perspectives, we studied the hierarchical organization of crypt epithelial cells in an unbiased fashion, by combining high-resolution, single-cell profiling and lineage tracing in multiple transgenic mouse models. These show that Lgr5 is not a specific ISC marker; rather, cells located in the crypt isthmus, which include Lgr5low cells, comprise the ISCs that sustain tissue homeostasis. Following irradiation or intestinal injury, surviving ISCs and progenitors, but not differentiated cells, participate in intestinal regeneration, suggesting that neither de-differentiation nor reserve stem cell populations are drivers of intestinal regeneration. Our results provide a novel viewpoint for the intestinal crypt epithelium, in which ISCs localize to the crypt isthmus, and ISC potential is restricted to stem and progenitor cells. ### Competing Interest Statement Dr. Califano is founder, equity holder, and consultant of DarwinHealth Inc., a company that has licensed some of the algorithms used in this manuscript from Columbia University. Columbia University is also an equity holder in DarwinHealth Inc. US patent number 10,790,040 has been awarded related to this work, and has been assigned to Columbia University with Dr. Califano as an inventor. [1]: #ref-1 [2]: #ref-2 [3]: #ref-3 [4]: #ref-4 [5]: #ref-7 [6]: #ref-8 [7]: #ref-11